Single-path color video projection systems employing corrective optics

ABSTRACT

A multimedia projector ( 100 ) includes a single-path frame-sequential color optical system in which light rays emitted by a light source ( 14 ) propagate through a color wheel ( 102 ) and an optical integrator ( 16, 120, 122 ), and are directed toward a transflective polarizing beam splitter ( 40 ) that separates them into P-polarized components ( 76 ) and S-polarized components ( 78 ). The P-polarized components are transmitted toward a reflective LCD ( 26 ) in which pixels in a dark state reflect the light rays without a polarization change and return them through the transflective polarizing beam splitter, whereas pixels in a bright state reflect the light rays with a 90° polarization change as S-polarized light rays ( 112 ), which are reflected by the transflective polarizing beam splitter toward a projection lens ( 27 ).

RELATED APPLICATION(S)

[0001] Not applicable

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable

TECHNICAL FIELD

[0003] This invention relates to color video projection display systemsand more particularly to light source and optical pathway componentsusable in single-path projection systems employing a reflective liquidcrystal on silicon (“LCOS”) light valve.

BACKGROUND OF THE INVENTION

[0004] Multimedia projection systems have become popular for purposessuch as conducting sales demonstrations, business meetings, andclassroom training. In typical operation, multimedia projection systemsreceive analog video signals from a personal computer and convert thevideo signals to digital information to control one or more digitallydriven light valves. Depending on the cost, brightness, and imagequality goals of the particular projector, the light valves may be ofvarious sizes and resolutions, be transmissive or reflective, and beemployed in single or multiple light path configurations.

[0005] Recently, more optimal sets of multimedia projectorcharacteristics have been achieved by employing reflective LCOS lightvalves. There are various optical architectures for employing reflectiveLCOS light valves. One employs a polarization beam splitter (“PBS”) cubeprism and a so-called Philips prism; another employs a PBS cube prism, adichroic prism, and spectrally selective wave plates; yet anotheremploys multiple PBS cube prisms; still another employs a PBS cube prismand tilted plates; and yet still another employs an off-axis designimplemented with linear polarizers, as opposed to PBS cube prisms. Foreach architecture, a number of variations exist, such as using crossedplates for color separation versus a solid “X-cube” prism colorseparator, using liquid filled PBS cubes instead of glass PBS cubes, andusing additional polarizers or wave plates. However, each of thesearchitectures is generally distinct from the others and from theinvention described herein.

[0006] All of the above architectures employ linear polarizedlight-sensitive devices for receiving light from a randomly polarizedlight source, reflecting the light off the LCOS light valves, andredirecting the reflected light, depending on its polarization directionor state, either out through a projection lens or back toward the lightsource. The polarization state of the light is determined by anelectronic image pattern applied to the LCOS light valve. To achieve adark state condition, selected LCOS light valve pixels do not change thepolarization of the reflected light, so the light returns to the lightsource and does not project toward the screen. To achieve a bright statecondition, selected LCOS light valve pixels rotate the polarizationdirection by 90°, so the light is directed through the projection lenstoward the screen. Projected image quality largely depends on how wellthe various optical path components establish, maintain, and analyze thelight polarization directions. Increased image brightness can beachieved by employing a multi path architecture and minimizing lightloss through the various optical path components. Image brightness isalso a function of the amount of collected light from the lamp and thecolor efficiency, which is generally lower for a single path opticalsystem.

[0007] In particular, the architecture employing a PBS cube prism and aPhilips prism is described in U.S. Pat. No. 5,777,789 for EFFICIENTOPTICAL SYSTEM FOR A HIGH RESOLUTION PROJECTION DISPLAY EMPLOYINGREFLECTION LIGHT VALVES, in which a cube PBS allows only linearlypolarized light to propagate to a color splitting/combining prism. Afterreflecting from the light valves, the light is “analyzed” by the PBScube and redirected according to the polarization direction of theanalyzed light. This architecture is disadvantageous because it requiressophisticated optical coatings and non-standard prism angles and hasskew ray depolarization caused by the PBS cube prism, stressbirefringence caused by long path lengths in glass elements, andconsiderable weight due to the prisms.

[0008] In the architecture employing a PBS cube prism, a dichroic prism,and spectrally selective wave plates, linearly polarized light is firstincident on a spectrally selective half-wave plate that changes thepolarization direction by 90° for one color band only. A PBS cubeseparates the rotated color band from the un-rotated color bands basedon their orthogonal polarization directions. Typically the green (“G”)band is selected as the rotated color band because a dichroic cubesplitter relatively easily separates the widely spaced wavelengths ofthe blue (“B”) and red (“R”) bands. After reflection from the lightvalves, the PBS cube analyzes the light, directs it according to itspolarization direction, and recombines the color bands. Because the PBScube has a non-ideal spectral response, a spectrally selective half-waveplate is required at the output face of the PBS cube so that all threecolor-bands have the same polarization direction after passing throughthe wave plate and can, therefore, all pass through a “clean-up”polarizer. This architecture is disadvantageous because of stressbirefringence caused by the large path lengths in glass, skew raydepolarization caused by the PBS cube prism, and considerable weight dueto the bulky prisms.

[0009] In the architecture employing multiple PBS cube prisms, light isseparated into R, G, and B light paths using dichroic filter plates.Each of the three color paths contains a PBS cube, and each PBS cubeallows only linearly polarized light to pass through to an associatedlight valve. Light reflected from the light valves is “analyzed” by therespective PBS cube and redirected according to the polarizationdirection of the analyzed light. For each color path, light propagatingtoward the projection lens is recombined with light from the other colorpaths via an X-cube prism. This architecture is disadvantageous becauseof considerable aggregate weight of the three PBS cube prisms and theX-cube prism, high component cost and complexity, stress birefringence,skew ray depolarization in the PBS cube prisms, and a large footprintcreated by the separated color paths.

[0010] In the architecture employing a PBS cube prism and tilted plates,the PBS cube prism allows only linearly polarized light to propagatetoward a set of tilted dichroic filter plates. The first plate reflectsone color band and passes the remaining light to the second dichroicfilter plate, where it is further split into two more color bands. Afterreflection from the light valves, the color bands of light retrace theirpaths and recombine via the color splitting plates. The light issubsequently “analyzed” by the PBS cube, and redirected according to thepolarization direction of the analyzed light. This architecture isdisadvantageous because the PBS cube prism is bulky, heavy, has stressbirefringence, and skew ray depolarization, and the projection lensrequires a long back working distance.

[0011] The architecture employing an off-axis design and linearpolarizers is described in “Projection Displays V,” SPIE Proceedings,January 1999, Vol. 3634, pp. 80-86. This architecture employs atwo-level arrangement in which the incoming light- propagates upwardlyat an angle and through crossed dichroic color splitting plates. A sheettype linear polarizer positioned in each color path polarizes the light.The polarized light continues to propagate upwardly and reflects off thelight valves. The polarization direction of the light is analyzed byanother sheet type linear polarizer in each color path. Light reflectedby dark state condition pixels undergoes absorption in the polarizer,and light reflected by bright state condition direction pixelspropagates through the polarizer to an X-cube prism color combiner. Thisarchitecture is disadvantageous because it has an unduly high-profile,two-level form factor and requires an proprietary, asymmetrical,off-axis projection lens.

[0012] What is still needed is a compact, light-weight, low-profilemultimedia projection system that achieves a bright, high-qualityprojected image at a relatively low cost.

SUMMARY OF THE INVENTION

[0013] An object of this invention is, therefore, to provide opticalarchitectures for providing bright, reflective LCD light valve-basedmultimedia projectors.

[0014] Another object of this invention is to provide multimediaprojectors that are lighter weight, more compact, potentially lesscostly, and of simpler optical design than prior projectors.

[0015] A further object of this invention is to provide multimediaprojectors having a single-path reflective light valve opticalarchitecture.

[0016] Still another object of this invention is to provide a higherefficiency illumination system for use in multimedia projectors.

[0017] The following descriptions of preferred embodiments of thisinvention refer to P-polarized light and S-polarized light. P-polarizedlight has a polarization pass orientation in the plane of incidence andreflection, and S-polarized light has a polarization pass orientationthat is parallel to the surface of an optical element, i.e., isorthogonal to the plane of incidence and reflection.

[0018] A first embodiment of a multimedia projector of this inventionincludes a color wheel-based frame-sequential color (“FSC”) opticalsystem in which polychromatic light rays emitted by a light sourcepropagate along an optical axis through the color wheel and an opticalintegrator. Diverging and randomly polarized FSC light rays exiting theoptical integrator are collimated by a first positive lens, reflected byan optional fold mirror, and directed toward a transflective polarizingbeam splitter and a reflective LCD. The FSC light rays striking thetransflective polarizing beam splitter are separated into P-polarizedcomponents and S-polarized components. The transflective polarizing beamsplitter transmits the P-polarized components toward the LCD andreflects the S-polarized components toward a wavelength-selective lightsensor that detects a predetermined color component transition andtransmits to a FSC controller a color wheel synchronization signal forproviding data to the LCD that corresponds to the color component beingpropagated through the color wheel. The P-polarized FSC light rays arereceived and reflected by the LCD with their polarization directionselectively changed or unchanged, depending on whether the pixels areswitched to a dark or a bright state condition. Pixels in the dark statecondition reflect the P-polarized FSC light rays without a polarizationdirection change and simply return through the transflective polarizingbeam splitter toward the light source. On the other hand, pixels in thebright state reflect the P-polarized FSC light rays with a 90°polarization direction change as S-polarized reflected FSC light rays,which are reflected by the transflective polarizing beam splitter anddirected toward a projection lens.

[0019] In a first alternative embodiment, the light-transmissionefficiency of the single-path projector is increased by inserting asingle polarization conversion prism assembly following the firstpositive lens. This causes a substantial majority of the FSC light raysto be P-polarized for transmission through the transflective polarizingbeam splitter. In a second alternative embodiment, the polarizationconversion prism is replaced by a polarization conversation assembly(“PCA”) based on either triangular or trapezoidal fundamental unitgeometry, which is essentially an array of small polarization conversionprisms.

[0020] A second embodiment of the invention increases thelight-transmission efficiency of the single-path projector by replacingthe optical integrator with first and second flyseye lens arrays. Acollimating lens is placed between the color wheel and the first flyseyelens, and a PCA and condenser lens are placed following the secondflyseye lens to focus the resulting quasi-uniformly polarized lightthrough the transflective polarizing beam splitter onto LCD 26.

[0021] A third embodiment of this invention simplifies the single-pathprojector by replacing the color wheel with a liquid crystal-basedcolor-switching device (color modulator) and placing it between thecondenser lens and the transflective polarizing beam splitter. Thisembodiment is further simplified by replacing the collimating lens witha collimating reflector in the light source and positioning the lightsource close to the first flyseye lens.

[0022] A fourth embodiment of this invention includes correction opticsto improve the light collection efficiency from the light source. Lightfrom an arc lamp is reflected by a reflector and focused through thecolor wheel by an aspherical corrector lens. The light emerging from thecolor wheel is re-collimated with an aspherical lens for propagationthrough the first and second flyseye lenses, the PCA, and the condenserlens. The aspherical reflector, the aspherical corrector lens and theaspherical lens coact to efficiently collect light from the arc, focusit through the color wheel with a minimal spot and collimate it throughthe integrator system. The angular spread of the beam at the entrance ofthe first integrator plate is minimized to match the acceptance anglesof the integrator and the multi-PBS to minimize the system Étendue. Theaspect ratio of the lenses of the first integrator plate matches thoseof the second integrator plate.

[0023] A fifth embodiment of this invention includes optics forimproving light collection efficiency when the light source is optimizedfor a small É tendue, which enables using reduced size opticalcomponents. Étendue is sometimes referred to as geometric extent orlight flux throughput. Étendue is important because in an optical systemit cannot be reduced without a reduction in light flux. It is ofparticular importance in the efficient collection of light flux from alight source, which effectively establishes the Étendue of the entireoptical system. Using a small É tendue for the system is important forkeeping the optics small (less costly). It is also a goal to maintain arelatively fast F# for the projection lens and other optical components,while still collecting the same amount of light. In this embodiment, toachieve suitable light collecting efficiency at a small Étendue, thelight source employs a double-paraboloid reflector having first andsecond focal points. Light rays are produced by an arc lamp that has itsarc located at the first focal point and a compound parabolicconcentrator (“CPC”) positioned at the second focal point. The lightrays enter the CPC over a wide range of acceptance angles but exit witha relatively small exit angle, which establishes the suitably smallÉtendue for the light source while propagating light having a high fluxdensity.

[0024] This invention is advantageous because it provides single-pathreflective LCOS light valve-based multimedia projectors that are lighterweight, more compact, potentially less costly, and of simpler opticaldesign than prior three-path projectors.

[0025] This invention is further advantageous because it providesimproved illumination systems that employ smaller optical components andincreases projected image brightness.

[0026] Additional objects and advantages of this invention will beapparent from the following detailed description of preferredembodiments thereof that proceed with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a simplified pictorial plan view of a multimediaprojector showing a first representative three-path reflective LCOSoptical system.

[0028]FIG. 2 is a simplified pictorial plan view of a multimediaprojector showing a second representative three-path reflective LCOSoptical system.

[0029]FIG. 3 is a simplified pictorial plan view of a multimediaprojector showing a first embodiment of this invention that furthersupports alternative polarization conversion assembly embodiments of asingle-path reflective LCOS optical system of this invention.

[0030]FIG. 4 is a simplified pictorial plan view of a multimediaprojector showing a second embodiment of a single-path reflective LCOSoptical system of this invention employing improved illuminationpolarization and integration.

[0031]FIG. 5 is a simplified pictorial plan view of a multimediaprojector of this invention showing a third embodiment of a single-pathreflective LCOS optical system employing improved illuminationefficiency and a color switching LCD device.

[0032]FIG. 6 is a simplified pictorial plan view of a multimediaprojector of this invention showing a fourth embodiment of a single-pathreflective LCOS optical system employing correction optics for improvedillumination efficiency.

[0033]FIG. 7 is a simplified pictorial plan view of a multimediaprojector of this invention showing a fifth embodiment of a single-pathreflective LCOS optical system employing a modified Cogent reflector andcompound parabolic concentrator for improved illumination efficiency atsmall Étendues.

[0034]FIG. 8 is a simplified pictorial cross-sectional view showing raytracings through a compound parabolic concentrator, such as the oneshown in FIG. 7.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0035] Commercially suitable single-path, reflective LCOS-based, colormultimedia projectors have been made possible by combinations of newlight sources, optical system, and LCOS device response times. Thedesign and advantages of these single-path projectors are betterappreciated by first considering representative three-path projectorsdescribed in copending U.S. patent application Ser. No. 09/535,427,filed Mar. 23, 2000, for COLOR VIDEO PROJECTION SYSTEM EMPLOYINGREFLECTIVE LIQUID CRYSTAL DISPLAY DEVICES, which is assigned to theassignee of this application.

[0036]FIG. 1 shows a first representative image projector 8 having athree-path optical system 10 constructed along an optical axis 12 andincluding a light source 14; a light pipe optical integrator 16 havingan inlet end 17 and an outlet end 18; a first positive lens 20; apolarization conversion prism assembly 22; an optional fold mirror 23; asecond positive lens 24; a three-path reflective LCD assembly 25 thatincludes first, second, and third reflective liquid crystal displays(“LCDs”) 26 ₁, 26 ₂, and 26 ₃ (collectively “LCD8s ”); and atelecentric-type projection lens 27.

[0037] Light source 14 includes an arc lamp 28 mounted at a focus of anelliptical reflector 29. An optional meniscus lens (not shown) may beplaced between elliptical reflector 29 and optical integrator 16 toconcentrate the light propagating from light source 14 and strikinginlet end 17. Optical integrator 16 is an elongated tunnel-typeintegrator with squared-off flat inlet and outlet ends 17 and 18. Inletand outlet ends 17 and 18 of optical integrator 16 have cross-sectionalaspect ratios that provide a projection display format that iscompatible with conventional SVGA and XGA display addressing formats.

[0038] First positive lens 20 receives and collimates light propagatingfrom outlet end 18 of optical integrator 16. Polarization conversionprism assembly 22 is of conventional construction, preferably includinga 45° rhomboid prism 30, a right-angle triangle prism 31, and ahalf-wave plate 32. Fold mirror 23 receives S-polarized light rays 34from polarization conversion prism assembly 22 and reflects them towardsecond positive lens 24, which receives the reflected S-polarized lightrays 34 and transmits them toward three-path reflective LCD assembly 25.(S-polarized light rays are indicated in the drawing figures by smallopen circles connected together by a line representing a lightpropagation path.) Because of glass, coating materials, and the Brewsterangle, polarizing conversion prism assembly 22 can also be optimized forangles between 37° and 55° and, of course, an optional cleanup polarizermay be positioned before three-path reflective LCD assembly 25.

[0039] The S-polarized light rays 34 are received by a spectrallyselective input wave plate 36, which transmits with polarizationdirection change a G range of light rays 34 to form P-polarized lightrays 38 and transmits without polarization direction change R and Branges of light rays 34. (P-polarized light rays are indicated in thedrawing figures by short-length transverse lines intersecting a linerepresenting a light propagation path.)

[0040] A plate-type transflective polarizing beam splitter 40 transmitsthe P-polarized G light rays 38 and reflects the S-polarized R and Blight rays 34. (Transflective polarizing beam splitters direct andrecombine all the wavelength ranges of incident light rays according totheir directions of polarization. S-polarized light rays are reflected,and P-polarized light rays are transmitted. ) P-polarized G light rays38 propagate through an optional field lens 42and impingetelecentrically on first LCD 261, and S-polarized R and B light rays 34impinge on a dichroic color filter 48, which divides them to form R andB light rays 44 and 46, which propagate through respective optionalfield lenses 42 ₂ and 42 ₃ and impinge telecentrically on respectivesecond and third LCDs 26 ₂ and 26 ₃. (Field lenses 42 ₁, 42 ₂, and 42 ₃are referred to collectively hereafter as “field lenses 42” and areoptional depending on the particular optical arrangement. )

[0041] The optical retarders, polarizers, wave plates, transflectivepolarizing beam splitters, dichroic filters, field lenses, and LCD lightvalves are available from a variety of manufacturers includingColorLink, Inc. of Boulder, Colorado; Moxtek, Inc. of Orem, Utah; andSharp Corporation of Nara, Japan. Transflective polarizing beam splitter40 is typically of a diffractive wire grid type, but acceptablealternatives include transflective polarizing beam splitters formed frommulti-layer thin films, cholesteric polymer liquid crystals, andlaminated polymer sheets. The latter type consist of laminating togethermultiple thin polymer sheets, each having a different index ofrefraction, such as “DBEF” sheets available from 3M.

[0042] Each of LCDs 26 includes an array of pixels that are individuallycontrollable by a controller 49 that receives video information fromanalog or digital signal sources, such as a personal computer.Controller 49 interprets the video information and conveys to LCDs 26pixel image patterns that control each pixel to reflect light in one oftwo orthogonal polarization directions depending on whether the pixel isswitched to a dark or bright state condition. Pixels in the dark statecondition reflect incident light rays without change in polarizationdirection, and pixels in the bright state reflect incident light rayswith a 90° rotation in polarization direction. Grey scale images mayalso be generated with LCDs 26 by employing methods in controller 49that vary according to the specific type of LCDs. The grey scale imagingmethods fall roughly into analog and digital classes. In analog LCDdriving schemes, grey scales are typically achieved by driving the LCDsto a level between the dark and bright state conditions to cause partialpolarization phase retardation in the LCD. In digital LCD drivingschemes, grey scales are typically achieved by employing pulse widthmodulation between the dark and bright state conditions. However, theoperation of this three-path projector might best be understood from thefollowing descriptions in which only the dark and bright pixel stateconditions are considered.

[0043] With respect to the pixel dark state condition, P-polarized Glight rays 38 impinging on dark state pixels of first LCD 26arereflected without change in polarization direction and return asP-polarized G light rays 38 along their original paths toward lightsource 14 through transflective polarizing beam splitter 40. S-polarizedR and B light rays 44 and 46 impinging on respective second and thirdLCDs 26 ₂ and 26 ₃ are reflected without change in polarizationdirection as S-polarized R and B light rays 44 and 46, are recombined bydichroic color filter 48, and return along their original paths towardlight source 14 by reflecting off transflective polarizing beam splitter40.

[0044] With respect to the bright state pixels, P-polarized G light rays38 impinging on bright state pixels on first LCD 26 ₁ are reflected witha 90° change in polarization direction as S-polarized G light rays 50that propagate toward transflective polarizing beam splitter 40.S-polarized G light rays 50 strike transflective polarizing beamsplitter 40, which reflects them toward projection lens 27. S-polarizedR and B light rays 44 and 46 impinging on respective second and thirdLCDs 26 ₂ and 26 ₃ are reflected with a 90° change in polarizationdirection as respective P-polarized R and B rays 52 and 54 that strikedichroic color filter 48, which recombines and transmits them throughtransflective polarizing beam splitter 40 toward projection lens 27.

[0045] S-polarized G light rays 50 and P-polarized R and B light rays 52and 54 are received by a spectrally selective output wave plate 56 thatchanges the polarization direction of S-polarized G light rays 50 intoalignment with the polarization direction of P-polarized R and B lightrays 52 and 54 to produce P-polarized G, R, and B light rays 58. A“clean-up” polarizer 60 positioned between spectrally selective outputwave plate 56 and projection lens 27 color balances light rays 58 bycorrecting for non-ideal light transmission and reflection responses oftransflective polarization beam splitter 40 affecting at least one ofthe G, R, or B modulated light output beams. Spectrally selective outputwave plate 56 aligns the polarization direction of light rays 58 so thatclean-up polarizer 60 will not block light in one of the G, R, and Bwavelength ranges. Spectrally selective output wave plate 56 andclean-up polarizer 60 cooperate to improve the color purity and may beomitted if color purity is not an issue.

[0046]FIG. 2 shows a representative second image projector 68 having athree-path optical system 70 that is constructed similarly to opticalsystem 10 but includes neither polarization conversion prism assembly 22nor spectrally selective input wave plate 36. Accordingly, randomlypolarized G, R, and B light rays 72 propagate toward a three-pathreflective LCD assembly 74 and strike transflective polarizing beamsplitter 40. The randomly polarized G, R, and B light rays 72 can beseparated into P-polarized components 76 and S-polarized components 78.Transflective polarizing beam splitter 40 transmits P-polarizedcomponents 76 of G, R, and B light rays 72 toward field lens 261 andreflects S-polarized components 78 of G, R, and B light rays 72 towarddichroic color filter 48. Dichroic color filter 48 transmits S-polarizedG and R light rays 80 toward field lens 26 ₂ and reflects S-polarized Blight rays 82 toward field lens 26 ₃.

[0047] There is associated with each of field lenses 42 an optionaldichroic trim filter coating 84 ₁, 84 ₂, and 84 ₃ (collectively “trimfilters 84”) placed at a convenient location in the light propagationpaths of respective LCDs 26 ₁, 26 ₂, and 26 ₃. Trim filters 84 may beformed on separate substrates, directly on the windows of LCDs 26, or onthe surfaces of field lenses 42 as shown. Trim filters 84 reflectselected wavelength ranges of light rays without changing theirpolarization directions, so the reflected light simply propagates inreverse direction along the same path toward light source 14 withoutreaching projection lens 27. Light rays having the desired colorwavelength range propagate through trim filters 84 for reflection offthe pixels of LCDs 26.

[0048] In particular, dichroic trim filter coating 84receivesP-polarized G, R, and B light rays 76, reflects the R and B light raysin reverse direction along the same propagation path toward light source14 and transmits the G light rays toward LCD 26 ₁. The G light rays arereceived and reflected by. LCD 26 ₁, with their polarization directionselectively changed or unchanged, depending whether the pixels areswitched to a dark or bright state condition. The polarizationdirections of light rays reflected by the pixels the light rays strikecauses the G light rays to either propagate toward light source 14 orreflect toward projection lens 27 as described for optical system 10.Likewise, dichroic trim filter coatings 84 ₂ and 84 ₃ placed in thepaths of LCDs 26 ₂and 26 ₃ have the same effect on the R and B lightrays. In this way each of LCDs 26 ₁, 26 ₂, and 26 ₃ receives andreflects the respective G, R, and B light rays. Dichroic trim filter 84₃ is not required if dichroic color filter 48 provides suitable Bfiltering characteristics.

[0049] The above-described three-path projectors represent aconsiderable improvement over conventional prism-based projectors, butare still unduly bulky, optically complex, costly, and difficult tomanufacture. This invention overcomes these problems with multipleembodiments of single-path, FSC multimedia projectors.

[0050]FIG. 3 shows a first embodiment of a basic single-path multimediaprojector 100 of this invention. Single-path projector 100 is a furthersimplification of three-path projector 68 of FIG. 2. However, projector100 further includes a color modulator, such as a liquid crystal-basedcolor switcher, or preferably a color wheel 102, a wavelength-selectivelight sensor 104, and replaces controller. 49 with an FSC controller106. Projector 100 does not require LCDs 26 ₂ and 26 ₃, field lenses 42₂ and 42 ₃, trim filters 84, dichroic filter plate 48, nor specificallyselective output wave plate 56. Clean-up polarizer 60 is optional asdescribed below.

[0051] Polychromatic light rays emitted by arc lamp 28 are converged byelliptical reflector 29 to propagate along optical axis 12 through colorfilter segments of color wheel 102 and optical integrator 16. Colorwheel 102 preferably includes R, G, B, and light-purplish filtersegments. Because the light from arc lamp 28 is typically greenish(deficient in red), the light-purplish (non-white) filter segmentproduces a more accurate white color point and overall color gamut formultimedia projector 100. Diverging FSC light rays exiting opticalintegrator 16 are collimated by first positive lens 20, reflected byoptional fold mirror 23, and directed toward transflective polarizingbeam splitter 40 and LCD 26 as randomly polarized FSC light rays 108.The light rays propagating along optical axis 12 may be furtherconditioned by optical components such as UV/IR filters, notch filters,or polarizers.

[0052] Randomly polarized FSC light rays 108 striking transflectivepolarizing beam splitter 40 are separated into P-polarized components 76and S-polarized components 78. Transflective polarizing beam splitter 40transmits P-polarized components 76 of FSC light rays 108 toward fieldlens 42 and reflects, and in some cases, scatters S-polarized components78 of FSC light rays 108 toward wavelength-selective light sensor 104.

[0053] FSC light rays 108 preferably include R, G, B, and “lightpurplish” color components that change as color wheel 102 rotates. Lightsensor 104 detects a predetermined color component or componenttransition and transmits to FSC controller 106 a color wheelsynchronization signal. FSC controller 106 employs the synchronizationsignal to provide LCD 26 with data corresponding to the color componentbeing propagated through color wheel 102. This color wheelsynchronization technique is described further in U. S. Pat. No.5,967,636 for COLOR WHEEL SYNCHRONIZATION APPARATUS AND METHOD, which isassigned to the assignee of this application.

[0054] Optional field lens 42 receives P-polarized FSC light rays 76 andpropagates them toward LCD 26. P-polarized FSC light rays 76 arereceived and reflected by LCD 26 with their polarization directionselectively changed or unchanged, depending on whether the pixels areswitched to a dark state or bright state condition. Pixels in the darkstate reflect P-polarized FSC light rays 76 without a polarizationdirection change as P-polarized reflected FSC light rays 110, whichpropagate through transflective polarizing beam splitter 40 and returntoward light source 14. On the other hand, pixels in the bright statereflect P-polarized FSC light rays 76 with a 90° polarization directionchange as S-polarized reflected FSC light rays 112, which are reflectedby transflective polarizing beam splitter 40 and directed towardprojection lens 27.

[0055] Clean-up polarizer 60 may be optionally employed to correct fornon-ideal response of transflective polarizing beam splitter 40, whichin practice reflects a small amount of P-polarized light along with theS-polarized light.

[0056] Skilled workers will understand that using tilted plate optics,such as transflective polarizing beam splitter 40, presents challengesin the design of efficient optical coatings. For example, pooranti-reflective coatings can produce “ghost” images that may reduce theoverall system contrast. Consideration should be given, therefore, tothe coating, placement, and orientation of optical components tominimize the possibility of such ghost reflections/images. In thisinvention, transflective polarizing beam splitter 40 is preferablyoriented with its active surface facing the light path between LCD 26and projection lens 27.

[0057] In a first alternative embodiment, the light-transmissionefficiency of single-path projector 100 can be increased by inserting apolarization conversion device, such as polarization conversion prismassembly 22 of FIG. 1, in optical axis 12 following first positive lens20. This causes a substantial majority of FSC light rays 108 to beP-polarized for transmission through transflective polarizing beamsplitter 40. The residual S-polarized FSC light rays are reflectedtoward light sensor 104 by transflective polarizing beam splitter 40.

[0058] In a second alternative embodiment of the invention, thepolarization conversion device can include a polarization conversationassembly (“PCA”) based on either triangular or trapezoidal fundamentalunit geometry. There are subtle performance differences between thesetypes of PCAs, and they can be assembled in. uni-directional orbi-directional configurations. However, both types of PCAs provide thesame basic functionality, given a nearly collimated input beam thatcontains a mixture of both S- and P-polarized light.

[0059] PCAs employ an immersed polarizing beam splitter that separatesthe two polarization states, such that the P-polarized light passesdirectly through the coating interface, while S-polarized light isreflected along an alternate path. The S-polarized light is convertedinto P-polarized light by a half-wave plate on the output face of thealternate path. Thus, substantially all of the light propagating from aPCA has the same polarization state. Skilled workers will recognize thatit is possible to construct a PCA that propagates substantiallyS-polarized light, and that single-path projector 100 can be adaptedaccordingly.

[0060]FIG. 4 shows a second embodiment of the invention in which thelight-transmission efficiency of single-path projector 100 is increasedby replacing optical integrator 16 with first and second flyseye lenses120 and 122, which function best with a collimated light source.Accordingly, a collimating lens 124 is placed between color wheel 102and first flyseye lens 120. A PCA 126 and a condenser lens 128 areplaced in the light path following first and second flyseye lenses 120and 122 to focus the uniform polarized light through transflectivepolarizing beam splitter 40 and field lens 42 onto LCD 26. Thisembodiment is simplified by eliminating optional fold mirror 23 of FIGS.1, 2, and 3. Otherwise the image projection functionality of thisembodiment is substantially the same as the FIG. 3 embodiments.

[0061]FIG. 5 shows a third embodiment of this invention in whichsingle-path projector 100 is simplified by replacing color wheel 102with a liquid crystal-based color-switching device 130 that provides therequired sequential light path switching through a predetermined set ofcolors. As in the FIG. 4 embodiment, this embodiment includes first andsecond flyseye lens arrays 120 and 122, PCA 126, and condenser lens 128,but does not include optional fold mirror 23. Color-switching device 130is preferably placed between condenser lens 128 and transflectivepolarizing beam splitter 40. This embodiment is further simplified byreplacing collimating lens 124 with a collimating reflector 132 in lightsource 14. Space is saved by placing light source 14 close to firstflyseye lens 120 and providing an intervening UV/IR filter 134 as a heatshield. Otherwise the image projection functionality of this embodimentis substantially the same as the FIG. 3 embodiments.

[0062]FIG. 6 shows a fourth embodiment of this invention in whichsingle-path projector 100 includes correction optics to improve lightcollection efficiency from light source 14. This embodiment includesfirst and second flyseye lens arrays 120 and 122, PCA 126, and condenserlens 128 of FIG. 4, but does not include optional fold mirror 23 orcollimating lens 124. Light from arc lamp 28 is reflected by anaspherical reflector 138 and focused through color wheel 102 by anoptional aspherical corrector lens 140. The light emerging from colorwheel 102 is re-collimated with an aspherical lens 142 for propagationthrough first and second flyseye lenses 120 and 122, PCA 126, andcondenser lens 128. Aspherical corrector lens 140 and asphericalcollimating lens 142 coact to efficiently collect light reflected byaspherical reflector 138, focus it through color wheel 102, and image itonto first flyseye lens 120 with minimum overfill. Preferably, the lightis imaged onto first flyseye lens 120 with an aspect ratio that matchesthe aspect ratio of LCD 26. Because overfill is reduced or eliminated,optical components downstream of aspherical collimating lens 142 may besmaller and, therefore, lighter and less costly. Otherwise the imageprojection functionality of this embodiment is substantially the same asthe FIG. 3 embodiments.

[0063] The illumination system of FIG. 6 is advantageous because itprovides increased light collection efficiency; allows using smaller,more closely spaced optical components; improves light homogeneity withfewer lens elements; and allows implementing a more compact projector.Of course, field lens 42 is once again optional.

[0064]FIG. 7 shows a fifth embodiment of this invention in whichsingle-path projector 100 includes optics for improving light collectionefficiency when light source 14 is optimized for a small Étendue, whichis described below. With the possible exception of light source 14, asmall Étendue enables using reduced size versions of all the opticalcomponents of FIG. 4.

[0065] The geometric entity, Étendue E, is defined as the product of thetransverse sectional area of a light beam and the divergence angle ofthe beam. Étendue is sometimes referred to as geometric extent or lightflux throughput.

[0066] The geometric entity Étendue E is represented mathematically as:${E = {{\int{\int{{\cos (\Phi)}{A}{\Omega}}}} = {{A\quad \Omega} = {{A\quad \pi \quad {\sin^{2}(\theta)}} = \frac{A\quad \pi}{4( {F\#} )^{2}}}}}},$

[0067] where Ω defines a cone of light diverging through across-sectional area A. Note that E is inversely proportional to thesquare of the f/#. Accordingly, if one desires a compact (inexpensive)optical system that has high brightness, E and A should be kept small toallow relatively small optics to inexpensively provide a suitably fastf/#. Étendue is important because in an optical system it cannot bereduced without a reduction in light flux. It is of particularimportance in the efficient collection of light flux from a lightsource, such as light source 14, which effectively establishes theÉtendue of the entire optical system.

[0068] For small Étendues a Cogent reflector system is more efficientthan conventional conic reflector systems. Using a small Étendue for thesystem is important for keeping the optics small (less costly) whilemaintaining a fast F# for the projection lens and other opticalcomponents.

[0069] In this embodiment, to achieve suitable light collectingefficiency (greater than 40%) at an Étendue (less than 7 mm²·st), lightsource 14 employs a variation of what is referred to as a Cogentreflector system. In particular, light source 14 employs a freeform,double-ellipsoid, or preferably a double-paraboloid reflector 150 havingfirst and second focal points 152 and 154. Light rays 155 are producedby an arc lamp 156 that has its arc located at first focal point 152 ofdouble-paraboloid reflector 150. Arc lamp 156 also includes a mirrorcoating 158 on its surface facing away from double-paraboloid reflector150 to reflect light rays back through the arc to join with and furtherintensify light rays 155.

[0070] Also referring to FIG. 8, a CPC 160 having input and output ends162 and 164 is positioned with input end 162 at second focal point 154of double-paraboloid reflector 150. Light rays 155 enter CPC 160 over awide range of acceptance angles 166 (up to 90°) but exit with arelatively small and constant exit angle 168 (preferably 30° or less),which establishes the suitably small Étendue for light source 14.

[0071] The light exiting CPC 160 and propagating through color wheel 102is collimated with collimating lens 124 for propagation through firstand second flyseye lenses 120 and 122, PCA 126, and condenser lens 128.Moreover, output end 164 of CPC 160 is shaped to match the aspect ratioof LCD. 26, which shape is imaged by collimating lens 124 with minimumoverfull onto first flyseye lenses 120. As in the other embodiments, theimage projection functionality of this embodiment is substantially thesame as the FIG. 3 embodiments.

[0072] The illumination system of FIG. 7 is advantageous because, itprovides increased light collection efficiency at small Étendues, whichallows a suitably fast optical system while employing smaller and,therefore, less costly optical components, most notably color wheel 102,transflective polarizing beam splitter 40, LCD 26 (a 1.27 centimeter(0.5 inch) LCD may be employed), and projection lens 27. Small opticalcomponents are particularly useful for producing compact, lightweightportable projectors.

[0073] Advantages of the single-path projectors of this inventioninclude lighter weight, smaller size, fewer and less costly components,and easier implementations than alternative approaches, such astriple-path and large-prism based projection systems. The projectors ofthis invention are lighter weight partly because prisms are notnecessarily required or, because of the small Étendues made possible bythis invention, much smaller prisms may be employed to obtain the samelight transmission efficiency.

[0074] Moreover, there are performance advantages. The projectors ofthis invention offer higher image contrast because transflectivepolarizing beam splitters and small prisms have reduced birefringenceissues, which are typically caused by residual or thermally inducedstresses within large glass prisms. Finally, this invention enablesimplementing optical projectors having a faster f/#, resulting in higherluminous efficiency and more lumens (brightness) on the screen. This isbecause of the higher light collection efficiency of the small Étenduesmade possible by this invention and because transflective polarizingbeam splitter 40 does not suffer from f/# limitations. For example,prior glass prisms were limited by optical coating designs to aboutf/2.5. However, the small Étendues of this invention allow using suchprisms at about f/2.0. This is particularly true for the projectorembodiments of FIGS. 6 and 7, in which transflective polarizing beamsplitter 40 can be replaced by polarization beam splitting prism.

[0075] Skilled workers will recognize that various other portions ofthis invention may be implemented differently from the implementationsdescribed above for preferred embodiments. For example, minor opticalpath variations and additions may be necessary to correct forastigmatism, color aberrations, and other optical distortions. Also, thewavelength ranges, filters, wave plates, and other optical componentsmay employ a wide variety of characteristics, mounting positions,spacings, dimensions, and aspect ratios that are suited to particulardisplays, such as rear projection, higher resolution, video only, andentertainment applications. UV and/or IR filters may be employed toprotect components from damaging heat and radiation. The light sourceand illumination system embodiments may be used with multi-path systemsas well as the single-path systems shown and described. Finally, thepreferred embodiments are described with reference to G, R, B, andpurplish colors, but the invention is readily adaptable to monochrome,grey-scale, and other color systems.

[0076] It will be obvious to those having skill in the art that manyother changes may be made to the details of the above-describedembodiments of this invention without departing from the underlyingprinciples thereof. The scope of this invention should, therefore, bedetermined only by the following claims.

1-18. (cancelled)
 19. An apparatus, comprising: a light source adaptedto emit light; an aspherical corrector lens adapted to receive the lightand to transmit the light to an aspherical collimating lens that isadapted to substantially collimate the light; and a light homogenizingdevice adapted to receive the light from the aspherical collimating lensand to substantially homogenize the light.
 20. The apparatus of claim19, wherein the light source is further adapted to emit polychromaticlight and the apparatus further comprises: a color modulation devicethrough which the aspherical corrector lens is adapted to focus thepolychromatic light, said color modulation device being adapted totransmit frame sequential color (“FSC”) light to the asphericalcollimating lens.
 21. The apparatus of claim 20, wherein the colormodulation device comprises a selected one of a color wheel or a liquidcrystal-based color switching device.
 22. The apparatus of claim 19,wherein the light source comprises: an aspherical reflector with a firstfocal point; and an arc lamp adapted to emit light at the first focalpoint.
 23. The apparatus of claim 19, wherein the light homogenizingdevice comprises a selected one of a flyseye lens array or an opticalintegrator.
 24. A system comprising: a light source adapted to emitlight; an aspherical corrector lens adapted to receive the light and totransmit the light to an aspherical collimating lens that is adapted tosubstantially collimate the light; a light homogenizing device adaptedto receive the light from the aspherical collimating lens and tosubstantially homogenize the light; a plate-type transflectivepolarizing beam splitter adapted to receive the light from the lighthomogenizing device, to transmit rays of the light polarized in a firstpolarization direction, and to reflect rays of the light polarized in asecond polarization direction; and a reflective light valve adapted toreceive the light polarized in the first polarization direction, thereflective light valve including pixels selectively switchable betweenfirst and second states such that pixels switched to the first statereflect the light in the first polarization direction back through thetransflective polarizing beam splitter, and pixels switched to thesecond state reflect the light in the second polarization direction forreflection off the transflective polarizing beam splitter as imagebearing light.
 25. The system of claim 24, wherein the reflective lightvalve has an aspect ratio, and in which the light transmitted throughthe light homogenizing flyseye lens array further has a cross-sectionalaspect ratio that substantially matches the aspect ratio of thereflective light valve.
 26. The system of claim 24, further comprising:a projection lens adapted to project the image bearing light.
 27. Thesystem of claim 26, further comprising: a light polarizing filter,disposed between the transflective polarizing beam splitter and theprojection lens, adapted to correct for non-ideal light transmission andreflection responses of the transflective polarizing beam splitter. 28.The system of claim 24, in which the plate-type transflective polarizingbeam splitter includes at least one of a group consisting of a wire griddevice, a multi-layer thin film device, a cholesteric polymer liquidcrystal device, and a laminated polymer sheet device.
 29. The system ofclaim 24, wherein the light source is further adapted to emitpolychromatic light and the system further comprises: a color modulationdevice through which the aspherical lens is adapted to focus thepolychromatic light, said color modulation device being adapted totransmit frame sequential color (“FSC”) light to the asphericalcollimating lens.
 30. The system of claim 29, wherein the colormodulation device includes a color wheel or a liquid crystal-based colorswitching device.